Electronic circuits, such as integrated microcircuits, are used in a variety of products, from automobiles to microwaves to personal computers. Designing and fabricating microcircuit devices typically involve many steps, known as a “design flow.” The particular steps of a design flow often are dependent upon the type of microcircuit, its complexity, the design team, and the microcircuit fabricator or foundry that will manufacture the microcircuit. Typically, software and hardware “tools” verify the design at various stages of the design flow by running software simulators and/or hardware emulators, and errors in the design are corrected or the design is otherwise improved.
Several steps are common to most design flows. Initially, the specification for a new circuit is transformed into a logical design, sometimes referred to as a register transfer level (RTL) description of the circuit. With this logical design, the circuit is described in terms of both the exchange of signals between hardware registers and the logical operations that are performed on those signals. The logical design typically employs a Hardware Design Language (HDL), such as the Very high speed integrated circuit Hardware Design Language (VHDL). The logic of the circuit is then analyzed, to confirm that it will accurately perform the functions desired for the circuit. This analysis is sometimes referred to as “functional verification.”
After the accuracy of the logical design is confirmed, it is converted into a device design by synthesis software. The device design, which is typically in the form of a schematic or netlist, describes the specific electronic devices (such as transistors, resistors, and capacitors) that will be used in the circuit, along with their interconnections. This device design generally corresponds to the level of representation displayed in conventional circuit diagrams. Preliminary timing estimates for portions of the circuit may be made at this stage, using an assumed characteristic speed for each device. In addition, the relationships between the electronic devices are analyzed, to confirm that the circuit described by the device design will correctly perform the desired functions. This analysis is sometimes referred to as “formal verification.”
Once the relationships between circuit devices have been established, the design is again transformed, this time into a physical design that describes specific geometric elements. This type of design often is referred to as a “layout” design. The geometric elements, which typically are polygons, define the shapes that will be created in various materials to manufacture the circuit. Typically, a designer will select groups of geometric elements representing circuit device components (e.g., contacts, gates, etc.) and place them in a design area. These groups of geometric elements may be custom designed, selected from a library of previously-created designs, or some combination of both. Lines are then routed between the geometric elements, which will form the wiring used to interconnect the electronic devices. Layout tools (often referred to as “place and route” tools), such as Mentor Graphics' IC Station or Cadence's Virtuoso, are commonly used for both of these tasks.
With a layout design, each physical layer of the circuit will have a corresponding layer representation in the design, and the geometric elements described in a layer representation will define the relative locations of the circuit device components that will make up a circuit device. Thus, the geometric elements in the representation of an implant layer will define the regions where doping will occur, while the geometric elements in the representation of a metal layer will define the locations in a metal layer where conductive wires will be formed to connect the circuit devices. Typically, a designer will perform a number of analyses on the layout design. For example, the layout design may be analyzed to confirm that it accurately represents the circuit devices and their relationships as described in the device design. The layout design also may be analyzed to confirm that it complies with various design requirements, such as minimum spacings between geometric elements.
Still further, the layout design may be modified to implement one or more resolution enhancement techniques (RET). These implementation processes will modify the layout design data, to improve the usable resolution of the reticle or mask created from the design in a photolithographic manufacturing process. One such family of resolution enhancement technique (RET) implementation processes is sometimes referred to as optical proximity correction or optical process correction (OPC) processes. An OPC process may add features such as serifs or indentations to existing geometric elements in the layout design data, to compensate for diffractive effects that will occur during a lithographic manufacturing process. For example, an optical proximity correction process may modify a polygon in a layout design to include a “hammerhead” shape, in order to decrease rounding of the photolithographic image at the corners of the polygon.
After the layout design has been finalized, it is converted into a format that can be employed by a mask or reticle writing tool to create a mask or reticle for use in a photolithographic manufacturing process. Masks and reticles typically are made using tools that expose a blank reticle or mask substrate to an electron or laser beam (or to an array of electron beams or laser beams). Most mask writing tools are able to only “write” certain kinds of polygons, however, such as right triangles, rectangles or other trapezoids. Moreover, the sizes of the polygons are limited physically by the maximum beam (or beam array) size available to the tool. Accordingly, larger geometric elements in the layout design, or geometric elements that are not right triangles, rectangles or trapezoids (which typically are a majority of the geometric elements in a layout design) must be “fractured” into the smaller, more basic polygons that can be written by the mask or reticle writing tool. This process sometimes is referred to as “mask data preparation.”
Once a layout design has been fractured into shots, then the fractured layout design data can be converted to a format compatible with the mask or reticle writing tool. Examples of such formats are MEBES, for raster scanning machines manufactured by ETEC, an Applied Materials Company, and various vector scan formats for Nuflare, JEOL, and Hitachi machines, such as VSB11 or VSB12. The written masks or reticles then can be used in a photolithographic process to expose selected areas of a wafer to light or other radiation in order to produce the desired integrated circuit devices on the wafer.
To meet the demand for more powerful microcircuits, designers have regularly increased the average density of devices in a conventional microcircuit. For example, the area that might once have contained 100 transistors may now be required to contain 1,000 or even 100,000 transistors. Some current microcircuit designs call for microcircuit devices to be packed so closely that it may be difficult to properly manufacture adjacent device components in a single lithographic process. For example, a current microcircuit design may specify a series of parallel conductive lines positioned so closely that a conventional mask writer cannot resolve the pitch between the lines.
To address this issue, the structures in a layer of a microcircuit device are now sometimes formed using two or more separate lithographic processes. This technique, referred to as “double patterning,” partitions a layout design into two or more groups of geometric elements, each of which then is used to form a complementary lithographic mask pattern. Thus, if a layout design calls for as single geometric element having a series of closely-spaced parallel connective lines, this geometric element may be partitioned into multiple geometric elements so that adjacent lines are actually formed by different masks in separate lithographic processes. A “separation directive” may be employed to identify the geometric element edges that should be imaged by different masks. A separation directive may be, for example, a polygon (such as a rectangle) abutting both of the adjacent edges that will be formed by separate masks.
With some designs, like a series of closely-spaced parallel connective lines, it may be relatively easy to determine which edges should be formed by different masks. With other, more complex designs, however, it is often more difficult to determine which edges should be formed by different masks. For example, some designs may have geometric elements with substantial edges in two orthogonal directions (e.g., multiple geometrical elements with intermixed vertical and horizontal lines). With these more complex designs, it may be difficult to determine which pairs of edges should be formed using different masks.
To address this complexity, some separation techniques use specified rules to determine which edges in a layout design should be separately formed. For example, a rule-based separation technique may insert a separation directive between any two adjacent edges that are within a minimum distance of each other. Rule-based separation techniques are difficult to implement, however, because the rules must be developed on a case-by-case basis. This typically requires creating and manufacturing multiple circuit designs, identifying defects in the manufactured circuits, and then defining rules corresponding to the identified defects. Moreover, rule-based separation techniques can be prone to over-separate a circuit design (i.e., by separating edges that could, in fact, be formed using the same mask in a photolithographic process).